428 



KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1906. 



the number of these ultimate products identified has 

 been recently much increased, the substances isolated 

 all belong to the class of amino-acids; that is, they con 

 tain the characteristic acid carboxyl group " COOH 

 and the amino group " .\'H2," or a derivati\e of this. 

 It has, however, been shown that in addition to the 

 mono- and di-aniino acids, examples of which were 

 already known, a new class of amino-acids in which 

 the nitrogen is bound up in a ring, is present; as in- 

 stances of these cyclic nitrogen-containing acids and 

 pyrrolidine carboxylic acid, isolated bv Fischer and r<'- 

 CH.,-CH, 



I " I " 

 presented bv the formula CH2 CH. COOH mav be 

 \^ 

 NH 

 cited, and histidine and tryptophane are other members 

 of this class. 



The presence of oxy-amino acids among these 

 cleavage products is of interest since these may form a 

 link between carbohydrate and proteid in the animal 

 organism. 



In all, the identity of about twenty of these products 

 is now established, but the number of these obtained 

 from any one proteid \aries considerably; from the de- 

 composition of casein, the chief proteid present in milk, 

 as many as sixteen of these acids have been isolated, 

 whilst the comparatively simple protamines obtained by 

 Kossel from the sperm of fishes give only three or four 

 of them. 



These amino-acids form, therefore, the starting point 

 of all attempts at synthesis. In order that the synthesis 

 may be successful, two conditions are requisite; in the 

 first place, the synthetic substance must possess the 

 properties characteristic of the proteids; it should give 

 certain colour-reactions, notably a pink or violet colour, 

 when solutions of copper sulphate and caustic soda are 

 added and known as the " biuret " test; it should be 

 similar in physical properties, and it should be broken 

 doun both by mineral acids and by the proteolytic 

 enzymes giving simple amino-acids. The second con- 

 dition is that the method of synthesis should lead to a 

 product the constitution of which may be easily eluci- 

 dated. Neither of these requirements was fulfilled by 

 the earlier attempts at synthesis; it is true that Curtius 

 as early as 1882 isolated his " biuret base " as the re- 

 sult of the spontaneous decomposition of ethyl glycine, 

 but its constitution was only determined two years ago 

 by its discoverer after it had been shown by Schwarz- 

 schild to be decomposable by the pancreatic enzyme, 

 trypsin. The only systematic attempts at synthesis are 

 those of Emil Fischer, who has succeeded in finding a 

 general method by which long chains of amino-acids 

 may be built up into complicated molecules of undoubted 

 chemical individuality and of determinate structure. The 

 method appears capable of wide application, and con- 

 sists essentially in treating the chloride of a halogen- 

 substituted amino-acid with an amino-acid ester. The 

 resulting ester is saponified, and the halogen atom 

 subsequently removed by treatment with strong aqueous 

 ammonia. 



This is represented bv the following equations : — 

 CH, CI. COCl-f NH.,. 'CH„ CONH. CH, COO Et 



— >- CH,, CICONH. CH,. CONH. CH. COO Et 



— ^ CH.CICONH.CH, CO. NHCHCOOH 



— >~ CH., NH,. CONH. CH, CONH CH, COOH 



These condensed amino-acids are termed by their 



discoverer " peptides," the number of amino-acids 



coupled up being signified by the prefix di, tri, tetra, 



&c. Many of the decomposition products of native 



proteids have been used as the building-stones in this 

 method; glycocoll, alanin, leucin, tyrosin, and several 

 others have been made to react with the chlorides of 

 halogen derixatives of different fatty acids, and a great 

 variety of products have thus been obtained, as many as 

 five acids having been built up into one large molecule. 

 The properties of these polypeptides vary with the radi- 

 cals employed; they show less disposition to crystallise 

 as the number of residues built up increases; they are 

 soluble in water, those produced from optically active 

 acids being most soluble; the}' are precipitated by the 

 re-agents used to precipitate proteids, many give the 

 biuret reaction, and if tyrosin be one of the acids used 

 in their synthesis Millon's reaction is also given. In 

 addition, those built up from tyrosin and leucin are 

 acted upon by trypsin, these acids being split off as in 

 the action of enzymes on proteids. The peptides show, 

 therefore, properties closely analogous to thosL' of the 

 proteids, and the manner in which they are built up 

 furnishes us with valuable evidence as to the probable 

 atomic structure of the proteids themselves. 



Substances similar to these synthetic polypeptides 

 have now been isolated by P'ischer from the products 

 of hydrolytic decomposition of proteids, and this 

 furnishes strong evidence that the method of combina- 

 tion in these compounds is closely allied to' that in the 

 proteids themselves. The h3'drolysis by enzymes or 

 acids consists essentially in the addition of the elements 

 of water ;md in the subsequent splitting up of the 

 proteid molecule. This decomposition is gradual, and 

 takes place in successive stages, smaller and smaller 

 molecules being split off. \'arious investigators have 

 brought forward different schemes by which this action 

 may be represented; most of these are open to dispute, 

 for much remains to be known as to the exact relation- 

 ships obtaining between these products. The proteid 

 goes into solution in the presence of alkali or acid in 

 the form of alkali or acid-albumen; this is followed by 

 the appearance of albumoses, and at a later stage 

 peptones are detected, all which substances give the 

 biuret test. Much discussion has raged round the num- 

 ber and nature of the albumoses and peptones formed. 

 By some investigators they are classified into two 

 groups, one of which is completely resistant to the 

 lurther action of enzymes, and remains, therefore, as an 

 -"nd product of enzyme digestion; the second group, on 

 the other hand, is readily attacked by these agents. It 

 has for long been known that proteids are much more 

 readily and completely broken down by the action of 

 acids than they are by the action of enz\mes. It has 

 now been shown that although the action of the latter 

 is very much slower, it may be nearly as complete as 

 that of acids. Fischer and Abderhalden carried out a 

 sei ies of experiments in which proteids were digested 

 with pancreatic ferment for as long as seven months; 

 at the end of this period hardly any trace of the biuret 

 reaction was observable in the solution. Since the 

 biuret test is characteristic of both albumoses and pep- 

 tones, it follows that both these substances had disap- 

 peared, and can no longer be regarded as end-products 

 of the action of enzymes, their power of resistance 

 being only one of degree. The solution thus obtained 

 by prolonged pancreatic digestion contained not only 

 the acids already identified, but, in addition, a com- 

 plicated polypeptide, giving a barely perceptible biuret 

 reaction, and decomposed by further hydrolvsis with 

 acids into six already isolated amino-acids. 



Other observers, notably, Zunz and Pfaundler, had 

 already pointed out that if one endeavours to trace 

 quantitatively the course of proteid decomposition by 



